Device for pre-lithiation of negative electrode and method for pre-lithiation of negative electrode

ABSTRACT

A device for pre-lithiation including a pre-lithiation reactor sequentially divided into an impregnation section, a pre-lithiation section, and an aging section. The pre-lithiation reactor accommodates a pre-lithiation solution through which a negative electrode structure is moved. A negative electrode roll is arranged outside the pre-lithiation solution, and the pre-movement negative electrode structure is wound. A lithium metal counter electrode is arranged in the pre-lithiation solution of the pre-lithiation section, and is arranged to be spaced apart from the negative electrode structure to face the negative electrode structure moving in the pre-lithiation solution. A charging and discharging unit is connected to the negative electrode structure and connected to the lithium metal counter electrode, wherein the lithium metal counter electrode is tilted and the a separation distance between the lithium metal counter electrode and the negative electrode structure gradually increases in the moving direction of the negative electrode structure.

TECHNICAL FIELD

This application claims the benefit of priority based on Korean PatentApplication No. 10-2020-0105346, filed on Aug. 21, 2020, and the entirecontents of the Korean patent application are incorporated herein byreference.

The present invention relates to an apparatus and method forpre-lithiating a negative electrode.

BACKGROUND ART

Recently, secondary batteries capable of charging and discharging havebeen widely used as energy sources of wireless mobile devices. Inaddition, the secondary battery has attracted attention as an energysource of an electric vehicle, a hybrid electric vehicle, etc., whichare proposed as a solution for air pollution of existing gasolinevehicles and diesel vehicles using fossil fuel. Therefore, the types ofapplications using the secondary battery are currently much diversifieddue to the advantages of the secondary battery, and it is expected thatthe secondary battery will be applied to many fields and products in thefuture.

Such secondary batteries may be classified into lithium ion batteries,lithium ion polymer batteries, lithium polymer batteries, etc.,depending on the composition of the electrode and the electrolyte, andamong them, the amount of use of lithium-ion polymer batteries that areless likely to leak electrolyte and are easy to manufacture is on theincrease. In general, secondary batteries are classified intocylindrical batteries and prismatic batteries in which an electrodeassembly is embedded in a cylindrical or rectangular metal can,depending on the shape of a battery case, and pouch-type batteries inwhich the electrode assembly is embedded in a pouch-type case of analuminum laminate sheet. The electrode assembly built into the batterycase is composed of a positive electrode, a negative electrode, and aseparator interposed between the positive electrode and the negativeelectrode, and is a power generating element capable of charging anddischarging. The electrode assembly is classified into a jelly-roll typewound with a separator interposed between the positive electrode and thenegative electrode which are long sheet-shaped and are coated withactive materials, and a stack type in which a plurality of positiveelectrodes and negative electrodes of a predetermined size aresequentially stacked while a separator is interposed therebetween.

The positive electrode and the negative electrode are formed by applyinga positive electrode slurry containing a positive electrode activematerial and a negative electrode slurry containing a negative electrodeactive material to a positive electrode current collector and a negativeelectrode current collector, to thereby form a positive electrode activematerial layer and a negative electrode active material layer,respectively, followed by drying and rolling them.

In the case of such a negative electrode, a passive film such as a solidelectrolyte interface (SEI) layer is formed on the surface of thenegative electrode during the initial charge. The passive filminterrupts injection of the organic solvent into the negative electrodeand suppresses decomposition reaction of the organic solvent, therebystabilizing the structure of the negative electrode, improving thereversibility of the negative electrode, and allowing the negativeelectrode to be usable. However, since the formation reaction of thepassive film is an irreversible reaction, the consumption of the lithiumions is caused, thereby decreasing the capacity of the battery, and asthe battery cycle is repeated, the lithium ions are consumed, therebycausing capacity reduction and cycle lifespan reduction.

As such, a method for forming a passive film on the surface of anegative electrode, preventing the capacity reduction and improvingcycle lifespan by pre-lithiating the negative electrode throughinserting lithium into the negative electrode is currently developed.

Such a pre-lithiation method includes a physical method of allowinglithium metal to directly contact the surface of the negative electrode,and a method of connecting lithium metal with the negative electrode andelectrochemically charging the negative electrode. In the case of theelectrochemically charging method, charging is performed in a state thatlithium metal is set to be spaced apart from a negative electrode by apredetermined distance. In this case, a uniform SEI film was not formedon the negative electrode due to the movement of the lithium metal andthe negative electrode in the electrolyte solution. In this case, theinitial efficiency and cycle characteristics of the negative electrodedecrease.

Hence, there is a need for a technology for enhancing the initialefficiency and cycle characteristics of a negative electrode byuniformly pre-lithiating the negative electrode.

DISCLOSURE Technical Problem

An object of the present invention is to provide a negative electrodepre-lithiation apparatus and method for enhancing initial efficiency ofthe negative electrode and preventing degeneration of a battery byuniformly pre-lithiating the negative electrode in pre-lithiating anegative electrode in an electrochemical charging scheme.

Technical Solution

An apparatus for pre-lithiating a negative electrode according to thepresent invention includes: a pre-lithiation reactor which issequentially divided into an impregnation section, a pre-lithiationsection and an aging section, and accommodates a pre-lithiation solutionin which a negative electrode structure is moved; a negative electroderoll which is arranged outside the pre-lithiation solution and on whichthe negative electrode structure before being moved is wound; a lithiummetal counter electrode which is arranged in the pre-lithiation solutionin the pre-lithiation section and is spaced apart from the negativeelectrode structure by a predetermined distance to face the negativeelectrode structure which is moved in the pre-lithiation solution; and acharge and discharge unit which is connected to the negative electrodestructure and the lithium metal counter electrode, wherein the lithiummetal counter electrode is tilted such that a separation distance withthe negative electrode structure gradually increases in a movingdirection of the negative electrode structure.

In a specific example, the lithium metal counter electrode is arrangedin the pre-lithiation section.

At this time, a separation distance between the lithium metal counterelectrode and the negative electrode structure at an ending point of thepre-lithiation section corresponds to 1.2 to 5 times of a separationdistance between the lithium metal counter electrode and the negativeelectrode structure at a starting point of the pre-lithiation section.

Further, the separation distance between the lithium metal counterelectrode and the negative electrode structure at the starting point ofthe pre-lithiation section is in a range of 1 to 20 mm.

Further, the negative electrode structure has a structure that anegative electrode active material layer is formed on at least onesurface of a negative electrode current collector, and a non-coated partis formed on at least one side in a width direction of the negativeelectrode active material layer.

At this time, the lithium metal counter electrode may be arranged toface only the negative electrode active material layer.

Further, the apparatus for pre-lithiating a negative electrode accordingto the present invention further includes a washing tank containing anorganic solvent.

Further, the apparatus further includes a drying unit which dries thenegative electrode structure having passed through the washing tank, anda collection roll for winding and unwinding the negative electrodestructure transferred to the drying unit.

Further, the present invention provides a method for pre-lithiating anegative electrode.

The method for pre-lithiating a negative electrode according to thepresent invention includes: preparing a negative electrode structure andthe above-described apparatus for pre-lithiating a negative electrode;impregnating the negative electrode structure with a pre-lithiationsolution while moving the negative electrode structure in theimpregnation section in the pre-lithiation reactor; pre-lithiating theimpregnated negative electrode structure while moving the negativeelectrode structure in the pre-lithiation solution of the pre-lithiationsection; and aging the pre-lithiated negative electrode structure in theaging section, wherein the pre-lithiating is performed by arranging alithium metal counter electrode, which is disposed to be spaced apartfrom the negative electrode structure, in a pre-lithiation section, andelectrochemically charging the negative electrode structure, and whereinthe lithium metal counter electrode is tilted such that a separationdistance with the negative electrode structure gradually increases in amoving direction of the negative electrode structure.

Herein, a separation distance between the lithium metal counterelectrode and the negative electrode structure at an ending point of thepre-lithiation section corresponds to 1.2 to 5 times of a separationdistance between the lithium metal counter electrode and the negativeelectrode structure at a starting point of the pre-lithiation section.

Herein, the separation distance between the lithium metal counterelectrode and the negative electrode structure at the starting point ofthe pre-lithiation section is in a range of 1 to 20 mm.

Further, the negative electrode structure has a structure that anegative electrode active material layer is formed on at least onesurface of a negative electrode current collector, and a non-coated partis formed on at least one side in a width direction of the negativeelectrode active material layer.

At this time, the lithium metal counter electrode may be arranged toface only the negative electrode active material layer.

Further, the method for pre-lithiating a negative electrode according tothe present invention further includes taking the negative electrodestructure out of the pre-lithiation reactor and washing the negativeelectrode structure.

Further, the method further includes drying the washed negativeelectrode structure.

Further, the present invention provides a method for manufacturing asecondary battery including the above-described method of pre-lithiatinga negative electrode.

Advantageous Effects

According to a method of pre-lithiating a negative electrode of thepresent invention, as the lithium metal counter electrode is tilted suchthat a separation distance with the negative electrode structuregradually increases in a moving direction of the negative electrodestructure, it is possible to uniformly pre-lithiate the negativeelectrode structure by fine current in the vicinity of the end in themoving direction of the lithium metal counter electrode, therebyimproving initial efficiency and cycle characteristics of a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of an apparatus forpre-lithiating a negative electrode according to the present invention.

FIGS. 2 to 4 are schematic diagrams illustrating an array relationshipbetween a negative electrode structure and a lithium metal counterelectrode in an apparatus for pre-lithiating a negative electrodeaccording to the present invention.

FIG. 5 is a flowchart illustrating the sequence of a method ofpre-lithiating a negative electrode according to the present invention.

FIG. 6 is a schematic diagram showing an array relationship between anegative electrode structure and a lithium metal counter electrodeaccording to comparative example 1.

FIG. 7 is a schematic diagram showing an array relationship between anegative electrode structure and a lithium metal counter electrodeaccording to comparative example 2.

FIG. 8 is a schematic diagram illustrating a state in which a negativeelectrode structure has been divided to measure initial efficiency foreach section, based on the width direction of the negative electrodestructure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the drawings. The terms and words used in the presentspecification and claims should not be construed as limited to ordinaryor dictionary terms and the inventor may properly define the concept ofthe terms in order to best describe its invention. The terms and wordsshould be construed as meaning and concept consistent with the technicalidea of the present invention.

In this application, it should be understood that terms such as“include” or “have” are intended to indicate that there is a feature,number, step, operation, component, part, or a combination thereofdescribed on the specification, and they do not exclude in advance thepossibility of the presence or addition of one or more other features ornumbers, steps, operations, components, parts or combinations thereof.Also, when a portion such as a layer, a film, an area, a plate, etc. isreferred to as being “on” another portion, this includes not only thecase where the portion is “directly on” the another portion but also thecase where further another portion is interposed therebetween. On theother hand, when a portion such as a layer, a film, an area, a plate,etc. is referred to as being “under” another portion, this includes notonly the case where the portion is “directly under” the another portionbut also the case where further another portion is interposedtherebetween. In addition, to be disposed “on” in the presentapplication may include the case disposed at the bottom as well as thetop.

Hereinafter, the present invention will be described in detail withreference to the drawings.

FIG. 1 is a schematic diagram showing the structure of an apparatus forpre-lithiating a negative electrode according to the present invention.FIGS. 2 to 4 are schematic diagrams illustrating an array relationshipbetween a negative electrode structure and a lithium metal counterelectrode in an apparatus for pre-lithiating a negative electrodeaccording to the present invention.

Referring to FIG. 1, the apparatus 1 for pre-lithiating a negativeelectrode according to the present invention includes: a pre-lithiationreactor 10 which is sequentially divided into an impregnation section 10a, a pre-lithiation section 10 b and an aging section 10 c, andaccommodates a pre-lithiation solution 30 in which a negative electrodestructure 20 is moved; a negative electrode roll 40 which is arrangedoutside the pre-lithiation solution 30 and on which the negativeelectrode structure 20 before being moved is wound; a lithium metalcounter electrode 50 which is arranged in the pre-lithiation solution 30in the pre-lithiation section 10 b and is spaced apart from the negativeelectrode structure 20 by a predetermined distance to face the negativeelectrode structure 20 which is moved in the pre-lithiation solution 30;and a charge and discharge unit 60 which is connected to the negativeelectrode structure 20 and the lithium metal counter electrode 50, inwhich the lithium metal counter electrode 50 is tilted such that aseparation distance with the negative electrode structure 20 graduallyincreases in a moving direction of the negative electrode structure 20.

Further, in the present invention, the direction, in which the negativeelectrode structure is moved in each section, is defined as the movingdirection, which is indicated as the x-axis direction. Further, thewidth direction of the negative electrode structure is a directionperpendicular to the moving direction and is indicated as the y-axisdirection.

As described above, in the case of a pre-lithiation method using aelectrochemical charging scheme, charging is performed in a state thatlithium metal is set to be spaced apart from a negative electrode by apredetermined distance. In this case, a uniform SEI film was not formedon the negative electrode due to the movement of the lithium metal andthe negative electrode in the electrolyte solution. In this case, theinitial efficiency and cycle characteristics of the negative electrodedecrease.

As such, according to the present invention, as the lithium metalcounter electrode is tilted such that a separation distance with thenegative electrode structure gradually increases in a moving directionof the negative electrode structure in the process of electrochemicallycharging a negative electrode, it is possible to uniformly pre-lithiatethe negative electrode structure by fine current in the vicinity of theend in the moving direction of the lithium metal counter electrode,thereby improving initial efficiency and cycle characteristics of abattery.

Hereinafter, the configuration of an apparatus for pre-lithiating anegative electrode according to the present invention will be describedin detail.

Referring to FIG. 1, an apparatus 1 for manufacturing a negativeelectrode of the present invention may be an apparatus for manufacturinga negative electrode by pre-lithiating a negative electrode structure,for example, pre-lithiating a negative electrode by using anelectrochemical charging scheme, and may be an apparatus formanufacturing a negative electrode using a roll-to-roll process.

Specifically, the pre-lithiation reactor 10 is a place where apre-lithiation solution 30 is accommodated, and impregnation,pre-lithiation reaction, and aging of the negative electrode structureare performed. The pre-lithiation reactor 10 is sequentially dividedinto an impregnation section 10 a, a pre-lithiation section 10 b, and anaging section 10 c. As such, the negative electrode structure 20, whichis unwound from the negative electrode roll 40, is inserted into thepre-lithiation solution 30 to thereby be moved in each section of thepre-lithiation reactor 10.

At this time, sections were not divided in the pre-lithiation reactor 10in a closed manner and were abstractly divided according to the positionof the negative electrode structure 20 in the pre-lithiation reactor 10and according to the process which was performed according to theposition of the negative electrode structure 20 in the pre-lithiationreactor 10. Specifically, the impregnation section 10 a, thepre-lithiation section 10 b, and the aging section 10 c were notphysically divided and were abstractly divided according to the processin which the negative electrode structure 20 was performed in thecorresponding section. As the negative electrode structure 20 is movedin the pre-lithiation reactor 10, the negative electrode structure 20 ispre-lithiated while passing through each section. The movement of thenegative electrode structure may be performed by a transfer roll in thepre-lithiation solution.

Further, the pre-lithiation solution 30 may contain a lithium salt andan organic solvent.

Specifically, the lithium salt may contain at least one selected fromthe group consisting of LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀,LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li,(CF₃SO₂)₂NLi, chloro boran lithium, low aliphatic carboxylic acidlithium, and 4 phenyl boric acid lithium.

Any organic solvent, which is commonly used in the related art, may beused as the organic solvent, but a high boiling point organic solventmay be preferably used to minimize the consumption of the electrolytesolution for pre-lithiation by evaporation during pre-lithiation.

For example, the organic solvent may contain at least one selected fromthe group consisting of a carbonate solvent and an ester-based solvent.The non-aqueous solvent may contain at least one selected from the groupconsisting of propylene carbonate (PC), ethylene carbonate (EC), diethylcarbonate (DEC), dimethylcarbonate (DMC), dipropylcarbonate (DPC),dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane,tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methylcarbonate(EMC), gamma butyrolactone (g-boutilactone), ethyl propionate, methylpropionate, but the present invention is not limited thereto.

Further, the pre-lithiation solution may further contain an additive,and the additive may include at least one selected from the groupconsisting of vinylene carbonate, vinylethylene carbonate,fluoroethylene carbonate, salicylic acid, LiBF₄, LITFSI (Lithiumbis(trifluoromethanesulfonyl)imide), LiBOB (Lithium bis(oxalato)borate),and LiODFB (Lithium difluoro(oxalato)borate).

Further, the temperature of the pre-lithiation solution may be 10 to 80°C., specifically 20 to 60° C., and more specifically 25 to 40° C. Whenpre-lithiation is performed in the above temperature range, lithium canbe smoothly diffused.

As the pre-lithiation solution 30 is accommodated in the pre-lithiationreactor 10, the pre-lithiation solution 30 is included in all of theimpregnation section 10 a, the pre-lithiation section 10 b, and theaging section 10 c.

The size, shape, etc. of the pre-lithiation reactor 10 may beappropriately designed in consideration of the impregnation,pre-lithiation, and aging degree of the negative electrode structure,and the moving distance of the negative electrode structure according tothe roll-to-roll process, etc.

Further, the size or length of the impregnation section 10 a, thepre-lithiation section 10 b, and the aging section 10 c may beappropriately designed in consideration of the electrolyte solutionimpregnation, pre-lithiation and aging degree of the negative electrodestructure 20. Specifically, the ratio of the lengths of the impregnationsection 10 a, the pre-lithiation section 10 b, and the aging section 10c may be 1 to 10:1:0.5 to 21, and preferably 1.5 to 5:1:1.8 to 10 forsmooth pre-lithiation.

Further, the apparatus 1 for pre-lithiating a negative electrodeaccording to the present invention includes a negative electrode roll 40on which the negative electrode structure 20 is wound. The negativeelectrode structure 20 may be wound on the negative electrode roll 40and then unwound from the negative electrode roll to thereby be insertedinto the pre-lithiation solution 30 in the pre-lithiation reactor 10.Any roll, which is commonly used in a roll-to-roll process, may be usedas the negative electrode roll 40.

The diameter, width, etc. of the negative electrode roll 40 may beappropriately designed in consideration of the thickness, amount, etc.of the wound negative electrode structure. For example, the diameter ofthe negative electrode roll 40 may be in the range of 3 to 50 cm, andspecifically in the range of 5 to 12 cm. The width of the negativeelectrode roll 40 may be in the range of 5 to 40 cm, and specifically inthe range of 10 to 20 cm.

The negative electrode structure 20 has a structure that a negativeelectrode active material layer 22 is formed on at least one surface ofa negative electrode current collector 21, and a non-coated part 23 isformed on at least one side in a width direction of the negativeelectrode active material layer 22. At this time, a negative electrodeslurry containing a negative electrode active material is applied and isthen dried and rolled to thereby form a negative electrode activematerial layer 22. The negative electrode slurry may further includeconductive materials and binders.

The sheet for the negative electrode collector generally has a thicknessof 3 to 500 micrometers. The negative electrode current collector is notparticularly limited as long as it has electrical conductivity withoutcausing chemical changes in the battery, and examples thereof includecopper, stainless steel, aluminum, nickel, titanium, sintered carbon,copper or stainless steel of which the surface has been treated withcarbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, orthe like. In addition, like the positive electrode current collector,fine unevenness can be formed on the surface to enhance the bondingforce of the negative electrode active material, and it can be used invarious forms such as a film, a sheet, a foil, a net, a porous body, afoam, and a nonwoven fabric.

The negative electrode active material may contain at least one selectedfrom the group consisting of a carbon-based active material and asilicon-based active material.

The silicon-based active material may impart excellent capacitycharacteristics to the negative electrode or secondary battery of thepresent invention and may contain a compound represented by SiO_(x)(0≤x<2). Since Sift does not react with lithium ions, lithium cannot bestored, and thus x is preferably in the above range. More preferably,the silicon-based oxide may be SiO. The average particle diameter (D₅₀)of the silicon-based oxide may be 1 to 30 μm, and preferably 3 to 15 μmin terms of reducing side reaction with the electrolyte solution whilemaintaining structural stability during charge/discharge. The averageparticle diameter D₅₀ may be measured using, for example, a laserdiffraction method.

The carbon-based active material may impart excellent cyclecharacteristics or battery lifespan performance to a secondary batteryor a negative electrode for a secondary battery of the presentinvention. Specifically, the carbon-based active material may contain atleast one selected from the group consisting of artificial graphite,natural graphite, hard carbon, soft carbon, carbon black, acetyleneblack, Ketjen black, super P, graphene and textile carbon, andpreferably at least one selected from the group consisting of artificialgraphite and natural graphite. The average particle diameter (D₅₀) ofthe carbon-based oxide may be 10 to 30 μm, and preferably 15 to 25 μm interms of reducing side reaction with the electrolyte solution whilemaintaining structural stability during charge/discharge.

Specifically, both the silicon-based active material and thecarbon-based active material may be used as the negative electrodeactive material in terms of improving both the capacity characteristicsand cycle characteristics. Specifically, the negative electrode activematerial may include the carbon-based active material and thesilicon-based active material in the weight ratio of 50:50 to 95:5, andpreferably in the weight ratio of 60:40 to 80:20.

The conductive material is usually added in an amount of 1 to 30% byweight based on the total weight of the mixture including the positiveelectrode active material. Such a conductive material is notparticularly limited as long as it has electrical conductivity withoutcausing a chemical change in the battery, and examples thereof includegraphite such as natural graphite and artificial graphite; carbon blacksuch as carbon black, acetylene black, Ketjen black, channel black,furnace black, lamp black, and summer black; conductive fibers such ascarbon fiber and metal fiber; metal powders such as carbon fluoride,aluminum and nickel powder; conductive whiskey such as zinc oxide andpotassium titanate; conductive metal oxides such as titanium oxide; andconductive materials such as polyphenylene derivatives and the like.

The binder is added in an amount of 1 to 30% by weight, on the basis ofthe total weight of the mixture containing the positive electrode activematerial, as a component that assists in bonding between the activematerial and the conductive material and bonding to the currentcollector. Examples of such binders include polyvinylidene fluoride,polyvinyl alcohol, carboxymethylcellulose (CMC), starch,hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone,tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrenebutylene rubber, fluorine rubber, various copolymers and the like.

The negative electrode structure 20 can be pre-lithiated by beingcharged and discharged by a charge and discharge unit 60 after beingconnected to a lithium metal counter electrode 50 to be described later.

Further, in the present invention, the lithium metal counter electrode50 may be disposed in the pre-lithiation solution 30 and be disposed tobe spaced apart from the negative electrode structure 20 by apredetermined distance to face the negative electrode structure 20, tothereby function as a counter electrode to the negative electrodestructure during electrochemical charge for pre-lithiation.Specifically, when the lithium metal counter electrode 50 ispre-lithiated by electrochemical charging, the lithium metal counterelectrode 50 may function as a lithium source which supplies lithiumions into the negative electrode structure 20. The lithium metal counterelectrode 50 may have a sheet form disposed to face the negativeelectrode structure 20.

The thickness of the lithium metal counter electrode 50 may beappropriately set in consideration of the pre-lithiation level, and mayspecifically be 10 to 500 μm, and more specifically be 40 to 200 μm.

The lithium metal counter electrode 50 can prevent a short circuitphenomenon which may occur by a direct contact between the negativeelectrode structure 20 and the lithium metal counter electrode 50 duringelectrochemical charge by being spaced apart from the negative electrodestructure 20.

The lithium metal counter electrode 50 is tilted such that a separationdistance with the negative electrode structure 20 gradually increases ina moving direction of the negative electrode structure 20. Likewise,uniform pre-lithiation of the negative electrode structure 20 ispossible by allowing the lithium metal counter electrode 50 to be tiltedin a specific direction.

Referring to FIGS. 2 to 4 together with FIG. 1, lithium may be allowedto enter into the inner side of the negative electrode structure 20 atthe initial part of the pre-lithiation section 10 b by allowing thelithium metal counter electrode 50 and the negative electrode structure20 to be disposed to be close to each other at the starting point of thepre-lithiation section 10 b. Thereafter, fine current, which is appliedto the negative electrode structure 20, may be formed by increasing theseparation distance between the lithium metal counter electrode 50 andthe negative electrode structure 20 at the ending point of thepre-lithiation section 10 b, and a uniform and stable SEI film may beformed on the surface of the negative electrode structure 20 by suchfine current, thereby maintaining a stable state after pre-lithiation.Further, when the negative electrode structure is moved in thepre-lithiation section 10 b, the negative electrode structure 20 may beshaken to some degree. In this case, the negative electrode activematerial layer 22 of a portion, which does not sufficiently face thelithium metal counter electrode 50 in the negative electrode structure20, may not be sufficiently pre-lithiated. In this case, the finecurrent can alleviate the degree of non-uniformity of the pre-lithiationaccording to the width direction. As a result, the initial efficiencyand cycle characteristics of the battery can be improved.

On the other hand, when the distance between the lithium metal counterelectrode and the negative electrode structure at the starting point ofthe pre-lithiation section is the same as the distance between thelithium metal counter electrode and the negative electrode structure atthe ending point of the pre-lithiation section, such fine current doesnot occur, and thus it is difficult to uniformly charge the negativeelectrode structure.

Further, in the case that the lithium metal counter electrode is tiltedsuch that a separation distance with the negative electrode structuregradually increases in a moving direction of the negative electrodestructure, a SEI film is formed on the surface as fine current isapplied at the starting point of the pre-lithiation section. Thereafter,even when the lithium metal counter electrode gets close to the negativeelectrode structure, lithium ions fails to flow in and side reactionoccurs on the surface.

The separation distance (d₂) between the lithium metal counter electrode50 and the negative electrode structure 20 at the ending point of thepre-lithiation section 10 b may correspond to 1.2 to 5 times,specifically 1.2 to 3 times and more specifically 2 to 3 times of theseparation distance (d₁) between the lithium metal counter electrode 50and the negative electrode structure 20 at the starting point of thepre-lithiation section 10 b. When the difference between the separationdistance between the lithium metal counter electrode 50 and the negativeelectrode structure 20 at the starting point of the pre-lithiationsection 10 b and the separation distance between the lithium metalcounter electrode 50 and the negative electrode 20 at the ending pointof the pre-lithiation section 10 b is within the above range, a stableSEI film may be formed as fine current is sufficiently formed in thenegative electrode structure 20.

Herein, the separation distance (d₁) between the lithium metal counterelectrode 50 and the negative electrode structure 20 at the startingpoint of the pre-lithiation section 10 b may be in a range of 1 to 20mm. Specifically, the separation distance (d₁) between the lithium metalcounter electrode 50 and the negative electrode structure 20 at thestarting point of the pre-lithiation section 10 b may be in the range of3 to 15 mm, and more specifically in the range of 6 to 12 mm. When theseparation distance (d₁) between the lithium metal counter electrode 50and the negative electrode 20 is in the above range, it is possible tosufficiently prevent an electrode short circuit phenomenon which mayoccur by a direct contact between the negative electrode structure 20and the lithium metal counter electrode 50, and lithium can be smoothlyinserted into the negative electrode structure 20 at the time ofpre-lithiation. Further, referring to FIG. 4, the lithium metal counterelectrode 50 is arranged to fact only the negative electrode activematerial layer 22. Namely, the lithium metal counter electrode 50 isarranged not to face the non-coated part 23 formed on two sides of thenegative electrode active material layer 22. This is to prevent lithiummetal from being precipitated on the non-coated part 23 as the lithiummetal counter electrode 50 faces the non-coated part 23.

To this end, the length in the width direction (y-axis direction) of thelithium metal counter electrode 50 may be the same as the length in thewidth direction of the negative electrode active material layer 22.Alternatively, the length in the width direction of the lithium metalcounter electrode may be set to be smaller than the length in the widthdirection of the negative electrode active material layer by reflectingthe degree to which the negative electrode structure is shaken duringmovement.

Further, referring to FIGS. 1 to 4, the negative electrode activematerial layer 22 is formed on only one surface of the negativeelectrode current collector 21, and the lithium metal counter electrode50 is disposed to face only one surface where the negative electrodeactive material layer 22 is formed. In the case that the negativeelectrode active material layer is formed on two surfaces of thenegative electrode current collector, two lithium metal counterelectrodes may be disposed to face the negative electrode activematerial layer on two surfaces.

Further, referring to FIG. 1, an apparatus 1 for pre-lithiating anegative electrode according to the present invention further includes awashing tank 70 including an organic solvent 71. The washing tank 70 isarranged independently from the pre-lithiation reactor 10 and can beprovided as a place for washing the negative electrode structure 20where pre-lithiation has been performed. To this end, a transfer roll,which transfers the negative electrode structure 20 from thepre-lithiation reactor 10 to the washing tank 70, may be arrangedbetween the pre-lithiation reactor 10 and the washing tank 70. As such,the negative electrode structure 20 is moved in the organic solvent 71in the washing tank 70, and impurities remaining in the negativeelectrode structure 20 may be removed. The organic solvent 71 does notcontain lithium salt, and the same one as the organic solvent used forthe above-described pre-lithiation solution may be used. Specifically,at least one selected from the group consisting of dimethyl carbonate(DMC), ethylmethyl carbonate (EMC), and ethylene carbonate (EC) may beused as the organic solvent.

The length of the washing tank 70 or the moving distance of the negativeelectrode structure 20 in the washing tank 70 may correspond to 0.1 to 5times, and preferably 0.5 to 2 times of the length of the pre-lithiationsection 10 b, and in this range, the remaining impurities of thenegative electrode structure may be smoothly removed.

Further, referring to FIG. 1, the apparatus for pre-lithiating anegative electrode according to the present invention further includes adrying unit 80 for drying a negative electrode structure 20 havingpassed through the washing tank 70, and a collection roll 90 for windingand unwinding the negative electrode structure 20 transferred to thedrying unit 80.

The drying unit 80 may be provided as a place where the negativeelectrode structure 20, which has passed through the pre-lithiationreactor 10 and the washing tank 70, is dried. A transfer roll, whichtransfers the negative electrode structure 20 from the washing tank 70to the drying unit 80, may be arranged between the pre-lithiationreactor 10 and the washing tank 70. In addition, the drying unit 80 mayinclude air or inert gas. The inert gas may be at least one selectedfrom the group consisting of Ar, N₂ and He.

The temperature of the drying unit 80 may be in the range of 10 to 80°C., specifically in the range of 20 to 60° C., and more specifically inthe range of 25 to 40° C. This temperature range is preferable in thatthe oxidation of the negative electrode structure can be prevented, andthe pre-lithiated state can be maintained in the range.

The length of the drying unit 80 or the moving distance of the negativeelectrode structure in the drying unit 80 may correspond to 0.1 to 5times and preferably 0.5 to 2 times of the length of the pre-lithiationsection. In this range, the organic solvent remaining in the negativeelectrode structure can be smoothly removed, and it is possible toprevent a damage to the negative electrode structure, which may occur asthe organic solvent remains in the negative electrode structure for along time.

The collection roll 90 may wind and unwind the negative electrodestructure transferred to the drying unit 80. The collection roll 90 mayperform the function of collecting or retrieving the negative electrodestructure which has been pre-lithiated, washed and dried. The collectionroll 90 may be the same as the above-described negative electrode roll.

<Method of Pre-Lithiating Negative Electrode>

The present invention provides a method of pre-lithiating a negativeelectrode using the above-described apparatus for pre-lithiating anegative electrode.

FIG. 5 is a flowchart illustrating the sequence of a method ofpre-lithiating a negative electrode according to the present invention.

Referring to FIG. 5, a method for pre-lithiating a negative electrodeaccording to the present invention includes: preparing a negativeelectrode and the above-described apparatus for pre-lithiating anegative electrode (S10); impregnating the negative electrode structurewith a pre-lithiation solution while moving the negative electrodestructure in the impregnation section in the pre-lithiation reactor(S20); pre-lithiating the impregnated negative electrode structure whilemoving the negative electrode structure in the pre-lithiation solutionof the pre-lithiation section (S30); and aging the pre-lithiatednegative electrode structure in the aging section (S40). At this time,the pre-lithiating is performed by arranging a lithium metal counterelectrode, which is disposed to be spaced apart from the negativeelectrode structure, in a pre-lithiation section, and electrochemicallycharging the negative electrode structure, and the lithium metal counterelectrode is tilted such that a separation distance with the negativeelectrode structure gradually increases in a moving direction of thenegative electrode structure.

According to the present invention, as the lithium metal counterelectrode is tilted such that a separation distance with the negativeelectrode structure gradually increases in a moving direction of thenegative electrode structure in the process of electrochemicallycharging a negative electrode, it is possible to uniformly pre-lithiatethe negative electrode structure by fine current in the vicinity of theend in the moving direction of the lithium metal counter electrode,thereby improving initial efficiency and cycle characteristics of abattery.

Referring to FIG. 5 together with FIGS. 1 to 4, a negative electrodestructure 20 is inserted into the pre-lithiating apparatus 1 asdescribed above. The negative electrode structure 20 has a structurewhere a negative electrode active material layer 22 is formed on atleast one surface of the negative electrode current collector 21, and anon-coated part 23 is formed on at least one side of the negativeelectrode active material layer 22, and is formed by applying a negativeelectrode slurry including a negative electrode active material to thenegative electrode current collector. The negative electrode structureis wound on the negative electrode roll.

Thereafter, the negative electrode structure 20 is unwound from thenegative electrode roll 40 and is inserted into the pre-lithiationreactor 10. First of all, the negative electrode structure 20 isinserted into the impregnation section 10 a in the pre-lithiationreactor 10 and is impregnated with the pre-lithiation solution 30 whilemoved.

At this time, the impregnation time may be appropriately set accordingto the pre-lithiation condition. For example, it may be 5 to 120minutes, specifically 10 to 90 minutes, and more specifically 15 to 40minutes. Through this, as the negative electrode structure becomessufficiently set in the pre-lithiation solution, the pre-lithiation maybe uniformly performed in the negative electrode structure. When theimpregnation time exceeds the above range, the durability of thenegative electrode structure decreases and the active material may beeasily detached from the current collector. When the impregnation timeis not within the range, it is difficult for the pre-lithiation solutionto be sufficiently permeated into the negative electrode structure andit may become difficult for the pre-lithiation to be uniformlyperformed.

Thereafter, the negative electrode structure 20 is pre-lithiated whilemoving in the pre-lithiation section 10 b where the lithium metalcounter electrode 50 is arranged. The pre-lithiating is performed byarranging a lithium metal counter electrode 50, which is disposed to bespaced apart from the negative electrode structure 20, in apre-lithiation section 10 b, and electrochemically charging the negativeelectrode structure 20 through a charge and discharge unit 60.

At this time, the lithium metal counter electrode 50 can prevent a shortcircuit phenomenon which may occur by a direct contact between thenegative electrode structure 20 and the lithium metal counter electrode50 during electrochemical charge by being spaced apart from the negativeelectrode structure 20.

Further, the lithium metal counter electrode 50 is tilted such that aseparation distance with the negative electrode structure 20 graduallyincreases in a moving direction of the negative electrode structure 20.Likewise, uniform pre-lithiation of the negative electrode structure 20is possible by fine current by allowing the lithium metal counterelectrode 50 to be tilted in a specific direction. The details aboutthis are as described above.

At this time, the separation distance (d₂) between the lithium metalcounter electrode 50 and the negative electrode structure 20 at theending point of the pre-lithiation section 10 b may correspond to 1.2 to5 times, specifically 1.2 to 3 times and more specifically 2 to 3 timesof the separation distance (d₁) between the lithium metal counterelectrode 50 and the negative electrode structure 20 at the startingpoint of the pre-lithiation section 10 b. When the difference betweenthe separation distance between the lithium metal counter electrode 50and the negative electrode structure 20 at the starting point of thepre-lithiation section 10 b and the separation distance between thelithium metal counter electrode 50 and the negative electrode 20 at theending point of the pre-lithiation section 10 b is within the aboverange, a stable SEI film may be formed as fine current is sufficientlyformed in the negative electrode structure 20.

Further, the separation distance (d₁) between the lithium metal counterelectrode 50 and the negative electrode structure 20 at the startingpoint of the pre-lithiation section 10 b may be in a range of 1 to 20mm. Specifically, the separation distance (d₁) between the lithium metalcounter electrode 50 and the negative electrode structure 20 at thestarting point of the pre-lithiation section 10 b may be in the range of3 to 15 mm, and more specifically in the range of 6 to 12 mm. When theseparation distance (d₁) between the lithium metal counter electrode 50and the negative electrode 20 is in the above range, it is possible tosufficiently prevent an electrode short circuit phenomenon which mayoccur by a direct contact between the negative electrode structure 20and the lithium metal counter electrode 50, and lithium can be smoothlyinserted into the negative electrode structure 20 at the time ofpre-lithiation.

Further, the lithium metal counter electrode 50 is arranged to face onlythe negative electrode active material layer 22. Namely, the lithiummetal counter electrode 50 is arranged not to face the non-coated part23 formed on two sides of the negative electrode active material layer22. This is to prevent lithium metal from being precipitated on thenon-coated part 23 as the lithium metal counter electrode 50 faces thenon-coated part 23.

Further, the electrochemical charging process for pre-lithiation can beperformed in the current density of 0.2 to 10 mA/cm², and preferably 2to 6 mA/cm². When the electrochemical charging is performed in thecurrent density of the above range, stable and uniform pre-lithiationcan be performed on the negative electrode active material.

After the negative electrode structure 20 is pre-lithiated in thepre-lithiation section 10 b, the negative electrode structure 20 is agedwhile passing through the aging section 10 c. Herein, the aging is astep of leaving the pre-lithiated negative electrode structure 20unattended in the pre-lithiation solution 30 for a predetermined time.

In this process, lithium ions inserted by pre-lithiation can be moreuniformly diffused to the inside and the surface of the negativeelectrode active material. If the aging step is not performed after thepre-lithiation, the lithium ions may not be uniformly diffused in thenegative electrode active material, thereby making it difficult tosufficiently remove the irreversible capacity, and there is apossibility that the uniform charge/discharge may not occur afterpreparation of the negative electrode.

Time, for which the negative electrode structure 20 is moved in theaging section 10 c, may correspond to 0.5 to 21 times, and preferably1.8 to 10 times of the time for which the negative electrode structure20 is moved in the pre-lithiation section 10 b. In this range, diffusionof lithium ions into the negative electrode active material can beuniformly performed, and it is possible to prevent a phenomenon that thenegative electrode active material layer is detached from the currentcollector due to the excessive aging or resistance increases due to theincrease in the thickness of the film on the surface of the negativeelectrode.

Further, the method for pre-lithiating a negative electrode according tothe present invention further includes taking the aged negativeelectrode structure 20 out of the pre-lithiation reactor 10 and washingthe negative electrode structure 20. Specifically, the negativeelectrode structure 20 is moved in the organic solvent 71 in the washingtank 70, and impurities remaining in the negative electrode structure 20may be removed. The organic solvent 71 does not contain lithium salt,and the same one as the organic solvent used for the above-describedpre-lithiation solution may be used.

Time, for which the aged negative electrode structure 20 is moved in thewashing tank 71, may correspond to 0.1 to 5 times, and preferably 0.5 to2 times of the time for which the negative electrode structure 20 ismoved in the pre-lithiation section 10 b. In this range, remainingimpurities of the negative electrode structure 20 can be smoothlyremoved.

The method for pre-lithiating a negative electrode according to thepresent invention further includes drying the washed negative electrodestructure.

The organic solvent remaining in the negative electrode structure by theimpregnation, pre-lithiation, aging and/or washing processes may beremoved by the drying step.

Specifically, the drying process can be performed by taking the washednegative electrode structure 20 out of the washing tank 70, insertingthe negative electrode structure 20 into a separately prepared dryingunit 80, and making the negative electrode structure 20 to be moved inthe drying unit 80.

The drying step may be performed by air or inert gas. Specifically, theinert gas may be at least one selected from the group consisting of Ar,N₂ and He.

The drying step may be performed in the range of 10 to 80° C.,specifically in the range of 20 to 60° C., and more specifically in therange of 25 to 40° C. This temperature range is preferable in that theoxidation of the negative electrode structure can be prevented, and thepre-lithiated state can be maintained in the range.

Time, for which the washed negative electrode structure 20 is dried, maycorrespond to 0.1 to 5 times and preferably 0.5 to 2 times of the timefor which the negative electrode structure is moved in thepre-lithiation section 10 b. In this range, it is possible to smoothlyremove the organic solvent remaining in the negative electrodestructure, and it is possible to prevent a damage to the negativeelectrode structure, which may occur as the organic solvent remains inthe negative electrode structure for a long time.

The collection roll 90 may be installed in the drying unit 80, and thenegative electrode structure 20 having moved in the drying unit 80 maybe wound by the collection roll 90. The negative electrode structure 20wound by the collection roll 90 may be cut into an appropriate size tothereby be finally manufactured as a negative electrode.

<Secondary Battery>

Further, the present invention provides a method for manufacturing asecondary battery including the above-described method of pre-lithiatinga negative electrode.

The secondary battery has a form where an electrode assembly, which hasa form that a separator is interposed between a positive electrode and anegative electrode, is accommodated in a battery case. The positiveelectrode has a structure that a positive electrode active materiallayer is formed as a positive electrode slurry containing a positiveelectrode active material is applied on a positive electrode currentcollector, and the negative electrode is as described above.

In the present invention, the positive electrode collector generally hasa thickness of 3 to 500 micrometers. The positive electrode currentcollector is not particularly limited as long as it has highconductivity without causing a chemical change in the battery. Examplesof the positive electrode current collector include stainless steel,aluminum, nickel, titanium, sintered carbon or aluminum or stainlesssteel of which the surface has been treated with carbon, nickel,titanium, silver, or the like. The current collector may have fineirregularities on the surface thereof to increase the adhesion of thepositive electrode active material, and various forms such as a film, asheet, a foil, a net, a porous body, a foam, and a nonwoven fabric arepossible.

In the present invention, the positive electrode active material is amaterial capable of causing an electrochemical reaction and a lithiumtransition metal oxide, and contains two or more transition metals.Examples thereof include: layered compounds such as lithium cobalt oxide(LiCoO₂) and lithium nickel oxide (LiNiO₂) substituted with one or moretransition metals; lithium manganese oxide substituted with one or moretransition metals; lithium nickel oxide represented by the formulaLiNi_(1−y)M_(y)O₂ (wherein M=Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga andcontains at least one of the above elements, 0.01≤y≤0.7); lithium nickelcobalt manganese composite oxide represented by the formulaLi_(1+z)Ni_(b)Mn_(c)Co_(1−(b+c+d))M_(d)O_((2−e))A_(e) such asLi_(1+z)Ni_(1/3)Co_(1/3)Mn_(1/3)O₂, Li_(1+z)Ni_(0.4)Mn_(0.4)Co_(0.2)O₂etc. (wherein −0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2,b+c+d<1, M=Al, Mg, Cr, Ti, Si or Y, and A=F, P or CO; olivine-basedlithium metal phosphate represented by the formulaLi_(1+x)M_(1−y)M′_(y)PO_(4−z)X_(z) (wherein M=transition metal,preferably Fe, Mn, Co or Ni, M′=Al, Mg or Ti, X=F, S or N, and−0.5≤x≤0.5, 0≤y≤0.5, 0≤z≤0.1).

Further, the positive electrode slurry further contains a conductivematerial and a binder as well as a positive electrode active material,which is as described above.

The separator is interposed between the positive electrode and thenegative electrode, and an insulating thin film having high ionpermeability and mechanical strength is used. The pore diameter of theseparator is generally 0.01 to 10 micrometers, and the thickness isgenerally 5 to 300 micrometers. Examples of such a separator includeolefin-based polymers such as polypropylene which is chemicallyresistant and hydrophobic; a sheet or a nonwoven fabric made of glassfiber, polyethylene or the like. When a solid electrolyte such as apolymer is used as the electrolyte, the solid electrolyte may also serveas a separator.

Further, the battery case is not particularly limited as long as it isused as an exterior material for packaging the battery, and acylindrical, square, or pouch type may be used and specifically apouch-type battery case may be used. The pouch-type battery case isgenerally made of an aluminum laminate sheet and may be composed of aninner sealant layer for sealing, a metal layer for preventing permeationof materials, and an external resin layer forming the outermost part ofthe case. Details of the battery case are known to those of ordinaryskill in the art, and thus detailed description thereof will be omitted.

When an electrode assembly is accommodated in a battery case, theelectrolyte solution is injected and sealed. Thereafter, a finalsecondary battery is manufactured through the formation process. Detailsabout the electrolyte solution are known to those of ordinary skill inthe art, and thus detailed description thereof will be omitted.

Hereinafter, the present invention will be described in detail withreference to examples. However, the embodiments according to the presentinvention may be modified into various other forms, and the scope of thepresent invention should not be construed as being limited to theexamples described below. The examples of the present invention areprovided to more fully describe the present invention to those skilledin the art.

Example 1

<Preparation of Negative Electrode Structure>

92 wt % of negative electrode active material (graphite 90%, SiO 10%), 3wt % of conductive material (Danka black), 3.5 wt % of binder (SBR), and1.5 wt % of thickener (CMC) was added to water to thereby prepare anegative electrode slurry.

The negative electrode slurry was coated on both surfaces of the coppercurrent collector (thickness: 8 μm), which was then rolled and dried ata vacuum oven of 130° C. for 12 hours, to thereby form a negativeelectrode active material layer on one surface of the copper negativeelectrode current collector (thickness: 70 μm, width: 10 cm) and formthe non-coated parts (1 cm) on two sides of the negative electrodeactive material layer, respectively, to thereby manufacture a negativeelectrode structure.

The negative electrode structure was wound on a negative electrode rollwhich is made of stainless steel and has a diameter of 3 inches.

<Preparation of Pre-Lithiation Reactor>

A pre-lithiation reactor made of stainless steel having width 270 cm xlength 20 cm x height 60 cm was prepared. The pre-lithiation solution ofthe amount corresponding to 30% of the height of the pre-lithiationreactor was injected into the pre-lithiation reactor. The temperature ofthe pre-lithiation reactor was maintained at 25° C.

The pre-lithiation solution was manufactured by adding LiPF₆ of 1.4Mconcentration as a lithium salt to an organic solvent, which is obtainedby mixing ethylene carbonate (EC), fluoroethylene carbonate (FEC) andethyl methyl carbonate (EMC) at the volume ratio of 10:20:70.

The pre-lithiation reactor was divided into an impregnation section, apre-lithiation section, and an aging section. A plurality of rolls wereinstalled in the impregnation section, the pre-lithiation section andthe aging section for smooth movement of the negative electrodestructure.

<Pre-Lithiation>

The negative electrode structure was unwound from the negative electroderoll, and was inserted into the pre-lithiation reactor at the speed of 1cm/min. The unwound negative electrode structure entered theimpregnation section and moved for 50 minutes to be impregnated with theelectrolyte solution.

The negative electrode structure having passed through the impregnationsection entered the pre-lithiation section, and electric current wasapplied at the current density of 3.04 mA/cm². At this time, the currentwas applied in pulse form of the period of 5 seconds.

A lithium metal counter electrode was disposed to be spaced apart fromthe negative electrode structure by a predetermined distance in thepre-lithiation section, and stainless steel (SUS) was joined to thelithium metal counter electrode to support the lithium metal counterelectrode.

At this time, the lithium metal counter electrode is tilted such that aseparation distance with the negative electrode structure graduallyincreases in a moving direction of the negative electrode structure.Specifically, the separation distance between the lithium metal counterelectrode and the negative electrode structure at the starting point ofthe pre-lithiation section was set to 6 mm, and the separation distancebetween the lithium metal counter electrode and the negative electrodestructure at the ending point of the pre-lithiation section was set to7.2 mm. Namely, a separation distance between the lithium metal counterelectrode and the negative electrode structure at an ending point of thepre-lithiation section was set to correspond to 1.2 times of aseparation distance between the lithium metal counter electrode and thenegative electrode structure at a starting point of the pre-lithiationsection.

The negative electrode structure having passed through thepre-lithiation section then entered the aging section and was aged whilemoving in the aging section for 30 minutes.

<Washing and Drying>

A washing tank made of stainless steel having width 50 cm x length 20 cmx height 60 cm was prepared. A roll for transferring a negativeelectrode structure was installed between the pre-lithiation reactor andthe washing tank. Dimethyl carbonate of the amount corresponding to 30%of the height of the washing tank was contained in the washing tank.

The aged negative electrode structure was taken out of thepre-lithiation reactor and was inserted into the washing tank, and movedin the washing tank for 50 minutes.

Thereafter, a drying unit made of stainless steel having width 20 cm xlength 20 cm x height 60 cm was prepared. The temperature of the dryingunit was 25° C., and inert gas was filled therein. A roll fortransferring the negative electrode structure was installed between thewashing tank and the drying unit. A collection roll was installed in thedrying unit.

The washed negative electrode structure moved in the drying unit throughthe roll for 20 minutes. The negative electrode structure having movedin the drying unit was wound by the collection roll.

Example 2

The negative electrode structure was manufactured in the same manner asin the example 1 except that in the pre-lithiation section, the lithiummetal counter electrode is tilted such that a separation distance withthe negative electrode structure gradually increases in a movingdirection of the negative electrode structure, and the separationdistance between the lithium metal counter electrode and the negativeelectrode structure at the starting point of the pre-lithiation sectionwas set to 6 mm, and the separation distance between the lithium metalcounter electrode and the negative electrode structure at the endingpoint of the pre-lithiation section was set to 12 mm.

Example 3

The negative electrode structure was manufactured in the same manner asin the example 1 except that in the pre-lithiation section, the lithiummetal counter electrode is tilted such that a separation distance withthe negative electrode structure gradually increases in a movingdirection of the negative electrode structure, and the separationdistance between the lithium metal counter electrode and the negativeelectrode structure at the starting point of the pre-lithiation sectionwas set to 6 mm, and the separation distance between the lithium metalcounter electrode and the negative electrode structure at the endingpoint of the pre-lithiation section was set to 18 mm.

Comparative Example 1

As shown in FIG. 6, the lithium metal counter electrode was arranged tobe parallel to the negative electrode structure in the pre-lithiationsection. At this time, the distance between the lithium metal counterelectrode and the negative electrode structure was maintained as 6 mm.The negative electrode structure was manufactured as in the example 1except the above point.

Comparative Example 2

The negative electrode structure was manufactured in the same manner asin the example 1 except that in the pre-lithiation section, the lithiummetal counter electrode is tilted such that a separation distance withthe negative electrode structure gradually decreases in a movingdirection of the negative electrode structure as shown in FIG. 7, andthe separation distance between the lithium metal counter electrode andthe negative electrode structure at the starting point of thepre-lithiation section was set to 6 mm, and the separation distancebetween the lithium metal counter electrode and the negative electrodestructure at the ending point of the pre-lithiation section was set to 3mm.

Experimental Example

<Preparation of Lithium Secondary Battery>

The negative electrode structure, which was manufactured in the examplesand comparative examples, divided into 3 parts in the width direction toset sections (sections a to c), and the negative electrode was cut inthe coin cell size in each section. Namely, the section b is a centralportion in the width direction of the negative electrode structure, andthe sections a and c are two edge portions adjacent to the non-coatedpart on the basis of the width direction in the negative electrodestructure.

Thereafter, a negative electrode, and a lithium metal foil, which was acounter electrode to the negative electrode, were prepared, and apolyolefin separator were interposed therebetween. Thereafter, acoin-type half-cell was manufactured by injecting an electrolytesolution which is obtained by dissolving 1M LiPF₆ in a solvent obtainedby mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at thevolume ratio of 50:50.

<Initial Reversibility Test>

A reversibility test for the coin-type half-cell manufactured asdescribed above was performed using an electrochemical charge-dischargedevice. During charging, the battery was charged at a current density of0.1 C-rate up to a voltage of 0.005 V (vs. Li/Li+), and discharged to avoltage of 1.5 V at the same current density during discharging. At thistime, the initial reversibility was measured using the ratio of thecharge capacity to the discharge capacity as in the Formula 1, and theresult was shown in Table 1.

Initial efficiency (%)={(initial discharge capacity)/(initial chargecapacity)}×100  [Formula 1]

TABLE 1 Initial Compar- Compar- efficiency Example Example Example ativeative (%) 1 2 3 Example 1 Example 2 Section a 99.4 99.0 98.3 94.3 89.5Section b 99.2 99.1 98.8 97.1 98.0 Section c 99.3 98.9 98.6 93.9 90.3

Referring to Table 1, it is seen that the initial efficiencies of thenegative electrode according to the examples was higher than those inthe comparative examples. This is because a lot of lithium ions enteredinto the negative electrode structure during the initial part of thepre-lithiation section, and then a stable SEI film was formed during thelatter part of the pre-lithiation section. On the other hand, in thecase of the comparative example 1, the initial efficiency was lower thanthat of the examples because fine current was not applied. In the caseof the comparative example 2, an SEI film was formed in a state that thecharge of lithium was not sufficient as fine current was applied duringthe initial part of the pre-lithiation, and as such, lithium failed toflow into the deep side of the negative electrode structure and sidereaction occurred on the surface, which lowered the initial efficiency.

Further, referring to Table 1, in the case of the negative electrodeaccording to the example, there was no significant difference in theinitial efficiency between sections because pre-lithiation was uniformlyperformed due to fine current. In the case of comparative examples 1 and2, as the negative electrode structure is shaken in the width directionwhile moving in the pre-lithiation section, time, for which two sides(section a and section c) of the negative electrode structure face thelithium metal counter electrode, decreased, thereby decreasing theamount of pre-lithiation. Further, in this situation, as fine currentwas not formed, pre-lithiation was not uniformly performed, therebymaking a difference in initial efficiency by sections.

The above description is merely illustrative of the technical idea ofthe present invention, and those skilled in the art to which the presentinvention pertains may make various modifications and variations withoutdeparting from the essential characteristics of the present invention.Therefore, the drawings disclosed in the present invention are notintended to limit the technical idea of the present invention but todescribe the present invention, and the scope of the technical idea ofthe present invention is not limited by these drawings. The scope ofprotection of the present invention should be interpreted by thefollowing claims, and all technical ideas within the scope equivalentthereto should be construed as being included in the scope of thepresent invention.

On the other hand, in this specification, terms indicating directionssuch as up, down, left, right, before, and after are used, but it isobvious that these terms are for convenience of description only and maychange depending on the location of the object or the location of theobserver.

1. An apparatus for pre-lithiating a negative electrode, the apparatuscomprising: a pre-lithiation reactor sequentially divided into animpregnation section, a pre-lithiation section and an aging section,wherein the pre-lithiation reactor accommodates a pre-lithiationsolution in which a negative electrode structure is moved through thepre-lithiation reactor; a negative electrode roll arranged outside thepre-lithiation solution and on which the negative electrode structurebefore being moved through the pre-lithiation reactor is wound; alithium metal counter electrode present in the pre-lithiation solutionin the pre-lithiation section, wherein the lithium metal counterelectrode is spaced apart from the negative electrode structure by apredetermined distance to face the negative electrode structure moved inthe pre-lithiation solution; and a charge and discharge unit connectedto the negative electrode structure and connected to the lithium metalcounter electrode, wherein the lithium metal counter electrode is tiltedand a separation distance from the negative electrode structuregradually increases in a moving direction of the negative electrodestructure.
 2. The apparatus of claim 1, wherein the lithium metalcounter electrode is arranged only in the pre-lithiation section.
 3. Theapparatus of claim 2, wherein the separation distance between thelithium metal counter electrode and the negative electrode structure atan ending point of the pre-lithiation section corresponds to 1.2 to 5times of the separation distance between the lithium metal counterelectrode and the negative electrode structure at a starting point ofthe pre-lithiation section.
 4. The apparatus of claim 3, wherein theseparation distance between the lithium metal counter electrode and thenegative electrode structure at the starting point of the pre-lithiationsection is in a range of 1 mm to 20 mm.
 5. The apparatus of claim 1,wherein the negative electrode structure comprises a negative electrodecurrent collector and a negative electrode active material layer on atleast one surface of the negative electrode current collector, and anon-coated part on at least one side of the negative electrode currentcollector in a width direction of the negative electrode active materiallayer.
 6. The apparatus of claim 5, wherein the lithium metal counterelectrode faces only the negative electrode active material layer. 7.The apparatus of claim 1, further comprising a washing tank containingan organic solvent.
 8. The apparatus of claim 7, further comprising: adryer connected to the washing tank, wherein the dryer dries thenegative electrode structure having passed through the washing tank, anda collection roll for winding and unwinding the negative electrodestructure transferred to the drying unit.
 9. A method for pre-lithiatinga negative electrode, the method comprising: preparing a negativeelectrode structure and the apparatus for pre-lithiating the negativeelectrode according to claim 1; impregnating the negative electrodestructure with the pre-lithiation solution while moving the negativeelectrode structure through the impregnation section in thepre-lithiation reactor to form an impregnated negative electrodestructure; pre-lithiating the impregnated negative electrode structurewhile moving the negative electrode structure through the pre-lithiationsolution of the pre-lithiation section to form a pre-lithiated negativeelectrode structure; and aging the pre-lithiated negative electrodestructure in the aging section, wherein the pre-lithiating is performedby arranging the lithium metal counter electrode spaced apart from thenegative electrode structure, in the pre-lithiation section, andelectrochemically charging the negative electrode structure, and whereinthe lithium metal counter electrode is tilted and the separationdistance from the negative electrode structure gradually increases inthe moving direction of the negative electrode structure.
 10. The methodof claim 9, wherein the separation distance between the lithium metalcounter electrode and the negative electrode structure at an endingpoint of the pre-lithiation section corresponds to 1.2 to 5 times of theseparation distance between the lithium metal counter electrode and thenegative electrode structure at a starting point of the pre-lithiationsection.
 11. The method of claim 9, wherein the separation distancebetween the lithium metal counter electrode and the negative electrodestructure at a starting point of the pre-lithiation section is in arange of 1 mm to 20 mm.
 12. The method of claim 9, wherein the negativeelectrode structure comprises a negative electrode current collector anda negative electrode active material layer on at least one surface ofthe negative electrode current collector, and a non-coated part on atleast one side of the negative electrode current collector in a widthdirection of the negative electrode active material layer.
 13. Themethod of claim 12, wherein the aged lithium metal counter electrodeonly faces the negative electrode active material layer.
 14. The methodof claim 9, further comprising removing the negative electrode structurefrom the pre-lithiation reactor and washing the negative electrodestructure.
 15. The method of claim 14, further comprising drying thewashed negative electrode structure.
 16. A method for manufacturing asecondary battery including the method of pre-lithiating a negativeelectrode according to claim 9.